首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2171篇
  免费   515篇
  国内免费   1237篇
测绘学   31篇
大气科学   1665篇
地球物理   153篇
地质学   1206篇
海洋学   452篇
天文学   10篇
综合类   140篇
自然地理   266篇
  2024年   29篇
  2023年   68篇
  2022年   122篇
  2021年   124篇
  2020年   114篇
  2019年   148篇
  2018年   131篇
  2017年   96篇
  2016年   115篇
  2015年   116篇
  2014年   207篇
  2013年   178篇
  2012年   203篇
  2011年   208篇
  2010年   178篇
  2009年   186篇
  2008年   220篇
  2007年   239篇
  2006年   251篇
  2005年   194篇
  2004年   148篇
  2003年   136篇
  2002年   69篇
  2001年   87篇
  2000年   63篇
  1999年   50篇
  1998年   48篇
  1997年   42篇
  1996年   34篇
  1995年   27篇
  1994年   22篇
  1993年   20篇
  1992年   19篇
  1991年   12篇
  1990年   5篇
  1989年   2篇
  1988年   5篇
  1985年   1篇
  1984年   1篇
  1982年   1篇
  1981年   4篇
排序方式: 共有3923条查询结果,搜索用时 875 毫秒
111.
渤海湾表层沉积物重金属与污染评价   总被引:12,自引:2,他引:10  
用原子吸收法测定了渤海湾表层沉积物中重金属的含量,同时用2种评价方法对该海域底质环境进行了评价。结果表明:沉积物中汞、锌含量平均值超过标准,其余项目未发现超标;其分布特征由于受沿岸不同污染源的污染影响,重金属污染物难以形成一定规律的分布状态。同时该海域底质环境质量总指数已达到污染程度和很强的生态危害,其中主要污染元素是Hg,已达重污染,并形成极强的生态危害。  相似文献   
112.
利用海南岛加密自动站逐小时降水资料、ERA5再分析资料,对海南岛短时强降水日环流配置进行了天气学分型,并进一步探讨了各天气型下海南岛短时强降水的时空分布、环流形势和关键环境参数特征。结果表明:(1)海南岛短时强降水有明显的日变化特征,呈单峰型,主要出现在15:00—19:00。(2)海南岛短时强降水的天气型主要有南海低压槽、华南沿海槽、西南低压槽和冷锋型。(3)南海低压槽、华南沿海槽、西南低压槽和冷锋型短时强降水分别占37%,31%,16%和16%。南海低压槽和华南沿海槽型主要出现在7、8和9月;西南低压槽型除9月外,其余各月份均可能出现;冷锋型绝大多数出现在4、5月。(4)南海低压槽和华南沿海槽型整层湿度条件都较好,不稳定能量较大,垂直风切变较弱。西南低压槽型不稳定能量较大,湿度条件一般,垂直风切变较弱。冷锋型存在明显的上干下湿特征,垂直风切变最大,0~6 km风速差75%分位大于10 m/s,不稳定能量最小。  相似文献   
113.
Accumulation and distributions of aliphatic and polyaromatic hydrocarbons (PAHs) and heavy metals were measured in tissues of the clam Ruditapes philippinarum collected from 5 sites in Jiaozhou Bay, Qingdao, China. The concentrations of total aliphatic hydrocarbon and PAHs ranged from 570 to 2 574 ng/gdw (gram dry weight) and from 276 to 939 ng/gdw, in the most and least polluted sites, respectively. The bio-accumulation of hydrocarbons and PAHs in the clams appeared to be selective. Aliphatic hydrocarbons were predominantly represented by short chain (<nC23) n-alkanes, suggesting that petroleum hydrocarbons were likely the major contamination source. The selective uptake of 3 and 4 ring PAHs, such as naphthalene, fluorene, phenanthrene, fluoranthene and pyrene, by the clams was probably related to the physiological and bio-kinetic processes that were energetically favorable for uptake of compounds with fewer rings. Accumulation of the metals Cd, Cu, Zn, Pb, Cr, Hg, and As in the clam tissues also showed high variability, ranging from 0.043 to 87 μg/gdw. Among the 7 detected metals, Zn, Cd, Cu, and As had a particularly high potential of accumulation in R. philippinarum. In general, a positive correlation was found between the tissue concentrations and sediment concentrations of hydrocarbons and of some metals. Our study suggests that moderate contamination with polyaromatic hydrocarbons, and low to moderate contamination with metals, currently exists for clam R. philippinarum in Jiaozhou Bay, in comparison with other regional studies. A long-term monitoring program is certainly needed for assessment of the potential ecological influence and toxicity of these contaminants of R. philippinarum in Jiaozhou Bay.  相似文献   
114.
平阴县土壤中重金属元素异常成因研究   总被引:1,自引:0,他引:1  
通过对平阴县周边及其西部农田区采样调查,平阴县周边元素异常具有异常衬度较高,富集系数大等特征,西部农田区异常具有各元素套合关系好,多数异常衬度不高,富集系数小的特点。结果表明,西部农田区异常是以As,Cd为主要成分的异常,元素含量与SiO2,Al2O3,Fe2O3等常量组分间具有显著的相关性,随SiO2含量的增加而降低,随Al2O3,Fe2O3含量的增加而增加,说明土壤常量组分及其所决定的土壤质地是导致异常形成的主要因素;而平阴城区异常是以S,Hg,As,Zn,Mn等元素为主的异常,是典型的人为综合因素(化工烟尘、工厂污水、生活垃圾)产生的异常。  相似文献   
115.
Contents of heavy metals, polycyclic aromatic hydrocarbons (PA Hs), dichlorodiphenyltrichloroethanes (DDTs) and hexachlorcy- clohexanes (HCHs) in surface sediments from mangrove areas of the Leizhou Peninsula were analyzed in July and November 2005. Risk assessment criteria applied by Long E R et al. (1995) and Long E D et al. (1995) (effects range low, ERL; effects range mean, ERM) of chemicals in sediments from the gulf or estuary were used to assess the potential ecological risks of heavy metals, PAHs, DDTs and HCHs to aquatic organisms in the studied area. The results indicated that the average contents of zinc, nickel, chromium, lead, copper, arsenic and mercury were (61.97 ± 55.87), (59.99 ± 39.01 ), (47.93 ± 28.37), (26.64± 13.00) , (23.45 ± 41.96), (9.32 ± 3.62), (0.14 ± 0.18) mg/kg in dry weight in the sediment samples col- lected from five studied sites in the Leizhou Peninsula, respectively. Cadmium was not calculated due to its content being below the detection limit ( 〈 0.3 mg,/kg). The average levels of Cr, Cu, Ni, Pb, Zn and Hg exceed their background values. The average contents of Ni were higher than ERM. The contents of PAHs in the sediments from the five studied sites were (79.78 ± 43.70) ng/g in dry weight, far lower than ERL(4 022 ng/g). The contents of DDE, DDD and DDTs in the sediments from five studied sites were (2.60 ± 4.68), ( 17.52 ± 27.25 ), (27.78 ± 46.64) ng/g in dry weight respectively, clearly higher than ERL, and the average contents of DDT were (7.66 ± 15.93) ng/g in dry weight, much higher than ERM. HCHs could be detected in the sediments only from Gaoqiao sampling site, with the average contents (0.07 ± 0.08) ng/g in dry weight.  相似文献   
116.
Historical sediment nutrient concentrations and heavy metal distributions were studied in four estuaries in the Gulf of Finland, Baltic Sea to examine the response of these estuaries to temporal changes in human activities. Cores were collected using a 1-m Mackereth corer and dated using 210Pb and 137Cs. The cores were analyzed for total carbon (TC), total nitrogen (TN), total phosphorus (TP), organic phosphorus (OP), inorganic phosphorus (IP), biogenic silica (BSi), loss-on-ignition (LOI), Cu, Zn, Al, Fe, Mn, K, Ca, Mg and Na. Principal component analysis (PCA) was used to summarize the trends in the chemical variables and to compare the trends at the different sites. Applying the 1986 137Cs date as a reference point, 210Pb chronologies were constructed for the sites using either the CRS model or a composite model (using both CIC and CRS). Significant increases were observed in sedimentation rates, TP and TN concentrations in all of the cores. Copper showed clear increases from 1850 towards present at all sites. Furthermore, redundancy analysis (RDA) was used to correlate environmental variables (catchment land use, catchment size, estuary surface area, depth and lake percentage) to sediment geochemistry. Based on redundancy analysis (RDA), the percentage of agriculture in the catchment was the most important factor affecting the sediment accumulation rate. Urban land-use types and industry correlate well with sediment Cu and Ca concentrations. Forest areas were related to high sediment BSi concentrations. Catchment land use was the most significant factor affecting sediment geochemical composition and sediment accumulation rates in these coastal embayments. Our results demonstrate that the coastal estuaries of the Gulf of Finland respond to the increased nutrient loading with the increased sedimentation and nutrient accumulation rates.  相似文献   
117.
综述了近10a国内外有关重金属、氨态氮、亚硝酸盐、有机污染物等几种环境因子对虾蟹类非特异性免疫影响的研究成果,以期为通过改善养殖环境条件提高虾蟹类自身免疫抗病力提供理论依据.同时,为进一步深入开展该领域的研究工作积累资料.  相似文献   
118.
3种经济螺体内4种重金属元素含量及评价   总被引:1,自引:0,他引:1  
以样品湿法消解消化法和火焰原子吸收光谱仪测定大口乳玉螺扁玉螺、亮螺的肌肉、肝脏、性腺3种器官组织中的As、Pb、Cd、Zn等4种金属元素的含量.分析不同种类的螺各器官重金属含量和重金属污染指数,评价各种螺的重金属污染反映其受污染程度和食用价值.结果表明:重金属在不同种类水产品不同组织器官中的分布不同;3种螺都受到了不同程度的重金属污染;3种螺的As都超过了“海洋生物污染评价标准”和“人体消费卫生标准”,其他的3种重金属含量也都不同程度地超过“两个标准”.  相似文献   
119.
通过对九龙江河口表层沉积物的重金属元素含量、沉积物粒度等要素进行分析,结合相关与聚类分析等统计手段,探讨了九龙江河口地区的沉积环境与重金属元素含量特征的关系.结果表明,即使在沉积环境复杂的河口地区,重金属含量分布与沉积物粒度仍存在明显的对应关系.受沉积动力影响,不同的重金属元素分布在九龙江河口地区表现出较大差异,而嵩屿至屿仔尾一带呈带状南北向展布的重金属相对高值条带值得引起注意.  相似文献   
120.
根据2003和2006年长江口及邻近海域表层沉积物样品的粒度组成和元素铬、铜、镍、铅、锌、铝、钙和锶的含量,分析了重金属元素含量的分布特征,探讨了含量的变化趋势及其对人类活动的响应。自2003年三峡工程一期蓄水完成到2006年6月,长江输沙量逐年减少,长江口海域沉积物中黏土的百分含量明显增加,但是沉积物分布的总体格局并没有发生明显的变化。重金属元素大多在泥质区沉积物中富集,高值区沿岸线呈带状分布,在最大浑浊带和口外羽状锋处达到峰值。在长江大量物质输入的背景下,研究区沉积物中重金属元素含量相对其他类似河口较低。河流的陆源颗粒输入、水动力条件、细颗粒物质的吸附以及絮凝作用是控制沉积物中重金属元素含量分布的主要因素,氧化还原条件也对重金属元素含量变化有一定的影响。人类活动(重大工程的建设和人为污染)对长江口外泥质区中重金属元素含量有重要影响,尤其对铅和锌的含量及其分布的影响显著。自2003年三峡工程一期蓄水以来,长江口海域表层沉积物中重金属元素沉积机制未有明显的变化,但是重金属元素含量有逐渐降低的趋势。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号